Luteinizing hormone (LH) and follicular stimulating hormone (FSH) are released from the anterior pituitary gland under tne control of the releasing hormone LH-RH produced in the hypothalamic region. LH and FSH act on the gonads to stimulate the synthesis of steroid hormones and to stimulate gamete maturation. The pulsatile release of LH-RH, and thereby the release of LH and FSH, controls the reproductive cycle in domestic animals and humans. Additionally, LH-RH has effects in placenta, in releasing HCG, and directly on the gonads. Agonist analogs of LH-RH are useful for the control of fertility by two mechanisms of action. Low doses of LH-RH analogs can stimulate ovulation and are useful in the treatment of hypothalamic and ovulatory infertility. Additionally they can be used for hypogonadal conditions and impotence, and stimulate spermatogenesis and androgen production in tne male. Paradoxically, larger doses of highly potent and long-lasting analogues of LH-RH have an opposite effect and block ovulation in the female and suppress spermatogenesis in the male. Related to tnese effects is a suppression of normal circulating levels of sexual steroids of gonadal origin, including reduction in accessory organ weight in the male and the female. In domestic animals this paradoxical effect promotes weight gain in a feed-lot situation, stimulates abortion in pregnant animals and in general, acts as a chemical sterilant.
The natural mammalian hormone releasing hormone LH-RH is a decapeptide comprised of naturally occuring amino acids (which have the L-configuration except for the achiral amino acid glycine). Its sequence is as follows: (pyro)Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH.sub.2. Many analogues of this natural material have been studied and the very large majority of them have proven to be of insufficient biological activity to be clinically useful. Certain select modifications have proven to have a beneficial effect on biological activity. By far the most significant modification is obtained by changing the 6-position residue from Gly to a D-amino acid. For example, replacing the Gly residue in the 6-position by D-Ala, D-Leu, D-Phe or D-Trp has led to a series of analogues of LH-RH with increased activity relative to LH-RH. M. Monahan, et al, Biochem., 12, 4616 (1973) for See [D Ala.sup.6 ]-LHRH; J. A. Vilchez-Martinez, et al, Biochem. Biophys. Res. Comm., 59, 1226 (1974) for [D-Leu.sup.6 ]LHRH and desGly.sup.10 [D-Leu.sup.6, Pro.sup.9 NHET.sup.10 ]LHRH; D. H. Coy, et al, J. Med. Chem., 19, 423 (1976) for [D-Phe.sup.6 ]LHRH; and W. Vale, et al, Clinical Endocrinology, 5th Supp., Blackwell Scientific Publications, Oxford, England (1976), p. 2615 and D. H. Coy, et al; Biochem. Biophys. Res. Comm., 67,576 (1979) for [D-Trp.sup.6 ]LHRH.
The structure of piscian (salmon) and avian (chicken) LHRHs are (pryo)-Glu-His-Trp-Ser-Tyr-Gly-Trp-Leu-Pro-GlyNH.sub.2 and (pyro)-Glu-His-Trp-Ser-Tyr-Gly-Leu-Gln-Pro-GlyNH.sub.2 respectively.
In addition to the substantial increases in activity obtained by the above-referred to substitutions in position 6, further increases in activity may be obtained by eliminating the Gly-NH.sub.2 in position 10 to afford a nonapeptide as an alkyl-, cycloalkyl- or fluoroalkylamide, or by replacing Gly-NH.sub.2 by an .alpha.-azaglycine amide. See for example, M. Fujino, et al, Biochem. Biophys. Res. Comm., 49, 863 (1972), D. H. Coy, et al, Biochem. 14, 1848(1975) and A. S. Dutta, et al, J. Chem. Soc. Perkin I, 1979, 379.
Substitution of N-methyl-leucine for the leucine residue in position 7 leads to increased stability towards enzymatic degradation. See for example, N. Ling, et al, Biochem Biophys. Res. Comm., 63, 801 (1975).
Substitution of the tryptophan residue in position 3 by 3-(1-naphthyl)-L-alanine leads to an increase in biological potency while 3-(2-naphthyl)-L-alanyl in this position leads to a substantial retention of activity. See for example, K. U. Prasad, et al, J. Med. Chem., 19, 492 (1976) and Y. Yabe, Chem. Pharm. Bull., 24 (12), 3149 (1976).
The tyrosine residue in position 5 can be replaced by phenylalanine or 3-(1-pentafluorophenyl)-L-alanine with the retention of substantial biological activity. See for example, N. Yanaihara, et al, Biochem. Biophys. Res. Comm., 52, 64 (1973), and D. Coy, et al, J. Med. Chem., 16, 877 (1973).
Although some polar 6 postion substituents retain substantial LHRH activity and in some cases are more potent that LHRH, the most potent analogues contain very hydrophobic 6 position substituents. Thus, while [D-LYS.sup.6 ]LHRH (potency 3.8 times LHRH), [D-Arg.sup.6 ]LHRH (potency 3.9 times LHRH), and [D-Arg.sup.6, Pro.sup.9 -NHEt]LHRH (potency 16.7 times LHRH) are active molecules, very hydrophobic analogues such as [D-Trp.sup.6 ]LHRH (potency 36 times LHRH) and [D-Trp.sup.6, Pro.sup.9 -NHEt]LHRH (potency 144 times LHRH) are dramatically more potent. See, for example, J. Rivier, et al., Peptides-Structure, Function, Biology, R. Walter and J. Meienhofer, Eds., Ann Arbor Science Publishers, Ann Arbor, MI (1975), p. 863, and W. W. Vale, et al., "Peptides-Structure and Biological Function", E. Gross and J. Meienhofer, Eds., Pierce Chan Co., Rockford, IL (1979), p. 781.
It would be desirable to prepare further analogues of native mammalian, piscian and avian LH-RH which have a high degree of biological activity but higher water solubility than that of those hydrophobic analogues heretofore described.
A series of novel amino acids with strongly basic side chain functionality have been prepared and incorporated into the native LH-RH sequence to yield agonistic LH-RH analogues. These novel amino acids yield highly potent analogues with greater water solubility than the very hydrophobic amino acids previously used to prepare the most potent LH-RH analogs. Some of these amino acids have been recently incorporated into antagonistic LHRH analogues (J. J. Nestor, Jr., et al, Eighth American Peptide Symposium, Tucson, AZ, May 22-27, 1983).